docs: def _photon_integrand(self, x, Egamma):
          """
          Integrand of Eq. 72
          """
          try:
              return (
                  self._sigma_inel(Egamma / x)
                  * self._particle_distribution((Egamma / x))
                  * self._Fgamma(x, Egamma / x)
                  / x
              )
          except ZeroDivisionError:
              return np.nan